The present invention relates to an optical semiconductor device, and a technique for producing the device, and is usable suitably for, for example, an optical semiconductor device including an optical directional coupler, and a method for producing the device.
Japanese Patent Application Laid-Open (JP-A) No. Hei 06 (1994) -059142 (Patent Literature 1) describes an optical directional coupler configured by making respective substantially central regions of first and second optical waveguides each formed over a surface of a silicon substrate closely near and parallel to each other over a predetermined length, thereby forming an optical coupling portion, this coupler having a structure in which a glass bank is formed outside each of the optical waveguides of the optical coupling portion, and the glass banks are parallel to each other at an interval of a desired distance and are equal in height to the optical coupling portion.
According to the optical directional coupler described in Patent Literature 1, when light rays having wavelengths different from each other, i.e., first and second wavelengths, respectively, advance from an input-side port of the first optical waveguide, the following is caused in the optical coupling portion: the first-wavelength light ray undergoes coupling between the optical waveguides one time to be emitted out from an output-side port of the second optical waveguide; and the second-wavelength light ray undergoes coupling between the optical waveguides two times to be emitted out from an output-side port of the first optical waveguide. In short, a light ray having a specified wavelength can be shifted from the first optical waveguide to the second optical waveguide.
However, for example, when the optical waveguides are formed to be decreased in line edge roughness (LER: the roughness of side-surfaces of the line pattern, or irregularities in the surfaces) in order to decrease a loss in light transmission in the optical waveguides (such a degree that signals attenuate in accordance with, for example, the transmission distance thereof), there is generated a phenomenon that in the optical coupling portion, a light ray having a specified wavelength is not easily shifted.